Revision as of 04:25, 16 July 2010

Outcomes

• Pupils should be able to recall and use the formula density = mass / volume
• Pupils should be able to take direct measurements for a range of regular objects and from these measurements find its density.
• Pupils should understand how the volume of an irregular object can be found by the displacement method using a measuring cylinder or by using a Eureka can. They should also appreciate how important it is to find the mass and volume of a multiple of small identical objects such as marbles.

Specification References

• 5.2 recall and use the relationship between density, mass and volume: density = mass/ volume
• 5.3 describe how to determine density using direct measurements of mass and volume

Starter

• DEMO from teacher.net
• 1. Principle: If you pack more mass into a the same volume, it's more dense.
• Common experience: going on vacation, suitcase is too full to close

Show an open hardtop suitcase that you're taking on vacation (full of clothes). Close it, then get a student volunteer to come up and lift it. Then open the suitcase, and talk about how you always get sucked into buying stuff while on vacation while tossing a bunch of tacky souveniers and other knickknacks you've bought on vacation in. Then ask another volunteer to sit on the suitcase to help cram it shut and do up the locks (you want it visibly too full, and for them to see things are getting compressed). Ask the original volunteer to lift the suitcase again. Is it heavier? (yes). Ask the class - did the mass change? (let them debate it out until they come up with a consensus of yes, it increased). Then ask the class if the volume changed (no, it's the same volume inside). Is it more or less dense? (at this point, only a few students will know it's more dense, so state it is more dense - if you put more mass into the same volume, it's more dense). Write this principle down on the board.

• 2. Principle: If you pack the same mass into a smaller volume, it's more dense.
• Common experience: crushing styrofoam (ie. everyone's put teethmarks on a styrofoam cup sometime or another)

Show a block of styrofoam. Get a hyper volunteer to jump on it until it's crushed a bit. Then ask the class if it's mass changed. (No, it'd still weigh the same on the scale). Then ask the class if it's volume changed. (Yes, it's squished, it's smaller now). Is it more or less dense? (let people hash it out awhile then say, yes, it's more dense, and write the second principle on the board).

• 3. Principle: Just because something has more mass doesn't mean it's more dense.
• Common experience: styrofoam packing chips, paper clip

Preparation: stuff a big, big plastic bag with styrofoam packing chips until, to you, it feels obviously heavier than a paper clip. Then get a student volunteer to hold the bag in one hand, and the paper clip in the other, and say which feels heavier. (If you have a strong bag, and some time to kill, it's fun to have the students toss the bag around the room at this point to get an idea of its weight). Now ask the students which is more dense, the paper clip or the bag of styrofoam chips. (At this point, I got an almost unanimous class vote for the paper clip since it was obviously metal). Then ask if you could assume something was more dense just because it had more mass. Write the third principle on the board. Ask how you could tell if some substance was denser than another substance. Try to avoid saying much here - let them debate it out and ask questions. (It depends on how much mass is packed into a volume - if you had the same volume of two things, then the one with more mass would be denser). At this point, the students ruminated & asked me case scenarios (ex. if I smushed the bag down to be really, really small - could it be denser than the paper clip).

• Summary: After the activity, ask students what they think the word density means. Get back answers and list point form as notes on the overhead. Restate and rewrite the three principles.

• Extend to: this is a nice time to show the formula and integrate with math and ratios now. Run through the three scenarios above, ask if mass or volume increased, if density increased. Ask if the numerator or denominator is changing in each scenario and what effect that has on the ratio. Speculate about 1. what sorts of things could happen in the numerator or denominator to increase a ratio 2. what sorts of things could happen in the numerator or denominator to decrease a ratio

Main Body of Lesson

• Density of Regular shapes - blocks and maybe a cylinder to challenge the more able. They can use the digital micrometer for one and a more sensitive balance. This will revise significant figures in measurements and in calculations.
• They can use their values to identify the material.
• You could also recap on prefixes - milli, centi, kilo along with conversions (1 ml = 1 cm³, m³ <--> cm³, g <--> kg) Conversion Questions for Density. Go through the quick way to convert between kg/m³ and g/cm³
• Density of Irregular shapes using Eureka Can and Displacement Method. The range of objects should also include pots containing 15-20 marbles so that pupils can understand the importance of measuring the volume and mass of multiple marbles.
• Density of Water using a measuring cylinder. It is important for pupils to know the density of water is 1g/cm³ or 1000kg/m³.
• If there is time I like to give them the densities of a few things so that they can check their anwers is plausable such as:

Plenary

• Here is the old Buried Treasure activity from Vince Robinson.
• Floating metal blocks and coins on mercury. This is an awesome one to show, the problem is that it involves using mercury. I'm currently looking into the Health and Safety aspects of this demo (PAS).
• Here are two nice videos about Sulphur Hexaflouride can be found here and here

Homework

• Read pages 16-21
• Q's 1-9 pg. 17
• Q's 1 and 2 pg. 19
• Extension Q's 1 and 2 pg. 20

Additional Information

Resources Required

• Density of Regular Shapes - cubes, blocks, cylinders, digital micrometers, balance, sensitive balance, rulers, copies of density table.
• Density of Irregular shapes - irregular shaped objects, eureka cans, balance
• Density of Water- measuring cylinders - range of sizes, balance
• Buried Treasure Activity - seven different sized platicine 'blobs' with a treasure (polystyrene ball painted gold?)hidden in two of them.

Textbook References

• None

Website References

• None

Skills Addressed

• Obtaining

Safety/Hazards

• General Laboratory Health and Safety Risk Assessment

Notes

• None